Historically, there have been essentially three approaches to generation of red, green and blue lasers. The first approach uses gas lasers, which have drawbacks such as high operating voltage, large footprint and short lifetime. The second approach employs second harmonic generation or sum frequency generation of infrared solid-state lasers. This approach is very complex with a high cost. In addition, laser performance may be instable, causing problems such as speckles in display systems. The third approach uses semiconductor laser diodes with or without frequency doubling. Unfortunately, the output powers of blue-green semiconductor lasers are limited due to degradation and optical damage of semiconductor laser materials under high electric stress and optical power density. If frequency doubling is involved, the system also shares the instability characteristic.
Upconversion fiber lasers (UFL), which provide for a reliable and efficient compact source of visible laser light at potentially low cost, are a promising alternative to the above-mentioned approaches. In conventional single-step optical pump processes, one pump photon is absorbed by one active ion for excitation to its upper laser level. Consequently, the laser emission always exhibits lower photon energy, i.e. longer wavelength, compared to the pump light. In contrast, the upconversion pump process is a multi-step process, in which more than one photons excite one active ion to the upper laser level and the laser light usually has a shorter wavelength than the pump light has. Common upconversion processes include excited-state absorption of a second photon by the active ion, also called “two-step absorption”, dipole-dipole cross-relaxation interactions between two excited ions (not necessarily the same atomic species), and avalanche absorption, which is a combination of both excited-state absorption and interionic cross-relaxation.
Upconversion fiber lasers using rare earth ion doped single-mode fluoride-glass fibers have been demonstrated in the visible wavelengths. In U.S. Pat. No. 5,067,134, Oomen described a diode-pumped thulium upconversion fiber laser operated at about 450 nm. In U.S. Pat. No. 5,226,049, Grubb described thulium and holmium co-doped upconversion fiber lasers, pumped by a laser diode emitting 1120 nm radiation or a Nd:YAG laser, to produce laser light of approximately 480 nm or 650 nm and 550 nm. Whitley et al., in Electronics Letters (1991), described an erbium upconversion fiber laser, pumped by an 801 nm laser diode and emitting 546 nm laser radiation. Piehler et al., in an article entitled “Green laser diode pumped erbium fiber laser” presented at the Compact Blue-Green Lasers conference 1994, described an erbium-doped fluoride fiber upconversion laser, pumped by a 971 nm laser diode through fiber coupling and operated at 544 nm. Piehler et al. also reported a praseodymium-doped fluoride fiber upconversion laser, simultaneously pumped by two laser diodes of different wavelengths to produce either 635 nm or 521 nm laser emission.
Single-mode rare-earth-ion doped fluoride-glass fibers are preferred media for upconversion. Because of their relatively slow vibrational decay rates, these materials have long excited-state lifetimes and a relative abundance of metastable intermediate states required for effective upconversion. Upconversion efficiency increases with pump intensity. Single-mode fibers (core diameter of 5 μm or less) can confine pump and laser radiations to a very small area over the length of these fibers (often several meters long), thus create very high optical intensities and large single-pass gains from only modest pump powers. This makes room temperature CW operation of an upconversion laser possible.
However, low melting point of fluoride glass fibers and the coupling losses caused by NA incompatibility between single mode fibers and high power laser diodes or laser diode arrays severely limit the power scaling in single clad, rare earth ion doped monomode fibers. In order to reduce the pump power density while scale-up the pump power, a larger fiber core diameter is needed. However, this will lead to multimode operation and, therefore, reduce beam quality.
To overcome these beam quality and power scaling related difficulties, specially configured double clad fluoride-glass fibers, which comprise a single mode core doped with rare earth ions and a surrounded inner cladding with a refractive index lower than the core index, have been proposed. The pump radiation is directed into the inner cladding area, which has an NA and cross section compatible with those of high power laser diodes or arrays. Multimode pump light from a high power diode laser array is converted to a single transverse mode laser output from a single mode core of the double clad fiber. Such fiber lasers can achieve high output powers. In U.S. Pat. Nos. 5,530,709 and 5,677,920, Waarts et al described an upconversion fiber laser using a double clad fiber structure. Zellmer et al, in Electronics Letters (1998), reported a Pr/Yb co-doped double-clad upconversion fiber laser operating at 635 nm. An output power of 440 mW was obtained when 3 W pump light at 840 nm was injected into a 25 μm inner pump cladding.
For projection display applications, much higher outputs at RGB wavelengths are required. In addition, multicolor visible laser light should be produced simultaneously or sequentially in a controllable manner. Furthermore, such systems have to be compact, low power consumption, stable, and low cost. These problems will be addressed in the present invention.